Problem: Solve for $k$ : $10 + k = 20$
Solution: Subtract $10$ from both sides: $ \begin{eqnarray} \\ 10 + k &=& 20 \\ \\ {-10} && {-10} \end{eqnarray} $ $ \begin{eqnarray} \hphantom{10 + k} &\hphantom{=}& \hphantom{20} \\ k &=& 20 {- 10} \\ \end{eqnarray} $ Simplify. $ k = 10$